Superconducting GHz Resonators for Microwave SQUID Multiplexing of Metallic Magnetic Calorimeters

Microwave SQUID multiplexing is the most promising way to read out large metallic magnetic calorimeter (MMC) detector arrays. Here, high quality superconducting GHz resonators are used for frequency encoding. The resonators are designed to have a bandwidth of $\sim$1 MHz to maintain the very fast signal rise time of MMCs. The frequency spacing between two neighbouring channels is set to $\sim$10 MHz to yield a crosstalk level below 10−4. The bandwidth of the resonators is adjusted by tuning the coupling capacitor for each resonator based on simulations of the couplers' electromagnetic properties. We investigate quarter-wave CPW transmission line (TLR) and lumped element resonators (LER) formed by a meander shaped inductor and an interdigital capacitor (IDC). The resonance frequency f${}_r$r of TLRs is set by adjusting the physical length of the resonator, whereas for LERs the number of IDC fingers is adjusted. Moreover, LERs allow to perform a post-production fine-tuning of the resonance frequency using a tile-and-trim process. We will present different aspects related to the fabrication, characterisation and optimisation of superconducting resonators to be used in a microwave SQUID multiplexer as well as our present tile-and trim process.